// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"

void freerange(void *pa_start, void *pa_end);

extern char end[]; // first address after kernel.
                   // defined by kernel.ld.

struct run {
  struct run *next;
};

// 修改1: 将kmem结构体改为数组形式，让每个CPU维护自己的空闲内存列表
// 原因: 消除锁冲突，提高并发性能
struct {
  struct spinlock lock;
  struct run *freelist;
} kmem[NPROC];

void
kinit()
{
  // 修改2: 原先初始化锁的地方改为初始化每个CPU的锁
  // 原因: 每个CPU维护自己的空闲内存列表，需要独立的锁
  for (int i = 0; i < NPROC; i++)
    initlock(&kmem[i].lock, "kmem");  // 初始化每个CPU的锁
  freerange(end, (void*)PHYSTOP);
}

void
freerange(void *pa_start, void *pa_end)
{
  char *p;
  p = (char*)PGROUNDUP((uint64)pa_start);
  for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE)
    kfree(p);
}

// Free the page of physical memory pointed at by v,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void
kfree(void *pa)
{
  struct run *r;
  // 修改3: 获取当前CPU ID，将页面释放到对应CPU的链表中
  // 原因: 实现每个CPU维护自己的空闲内存列表
  int id = cpuid();

  if(((uint64)pa % PGSIZE) != 0 || (char*)pa < end || (uint64)pa >= PHYSTOP)
    panic("kfree");

  // Fill with junk to catch dangling refs.
  memset(pa, 1, PGSIZE);

  r = (struct run*)pa;

  // 修改4: 使用对应CPU的锁，而不是全局锁
  // 原因: 消除全局锁竞争，提高并发性能
  acquire(&kmem[id].lock);
  r->next = kmem[id].freelist;
  kmem[id].freelist = r;
  release(&kmem[id].lock);
}

// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *
kalloc(void)
{
  struct run *r = 0;
  
  // 修改5: 遍历所有CPU的空闲列表，查找可用页面
  // 原因: 当当前CPU没有空闲页面时，可以从其他CPU获取
  for (int i = 0, curid = cpuid(); i < NPROC && !r; i++, curid++) {
    if (curid == NPROC)
      curid = 0;
    acquire(&kmem[curid].lock);
    r = kmem[curid].freelist;
    if (r) {
      kmem[curid].freelist = r->next;
    }
    release(&kmem[curid].lock);
  }

  if(r)
    memset((char*)r, 5, PGSIZE); // fill with junk
  return (void*)r;
}